In recent years, in order to obtain an electron beam having higher brightness, a Schottky electron emission source (hereinafter referred to also as an electron source) employing a needle electrode of tungsten single crystal, has been used. This electron source is one wherein a covering layer comprising zirconium and oxygen (hereinafter referred to as a ZrO covering layer) is formed on a tungsten single crystal needle having an axial orientation being <100> orientation, and by such a ZrO covering layer, the work function of the (100) crystallographic plane of tungsten single crystal is reduced from 4.5 eV to 2.8 eV. With this electron source, only the very small crystallographic facet corresponding to the (100) crystallographic plane formed at the apex of the needle becomes an electron emission area, whereby an electron beam having a higher brightness than by a conventional thermionic cathode can be obtained. It is known that this (100) crystallographic plane can be developed by applying a negative high voltage to the tungsten single crystal needle, i.e. by balancing the electrostatic stress by the high electric field with the surface tension at the apex of the needle. Further, this electron source has characteristics such that it has a long life, is more stable than a cold field emission cathode and is operable even with a high pressure and easy to use.
As shown in FIG. 1, in an electron source, a tungsten needle 1 having <100> orientation which emits an electron beam, is fixed by e.g. welding to a predetermined position of a tungsten filament 3 provided on conductive terminals 4 fixed to an insulator 5. A zirconium- and oxygen-supply source 2 is formed at a portion of the needle 1. Although not shown in the drawings, the surface of the needle 1 is covered with a ZrO covering layer.
The needle 1 is Joule heated by the filament 3 and used usually at a temperature of about 1,800 K. Accordingly, the ZrO covering layer on the surface of the needle 1 will be lost by evaporation. However, from the supply source 2, zirconium and oxygen will diffuse and will be continuously supplied to the surface of the needle 1, and consequently, the ZrO covering layer will be maintained.
In e.g. a scanning electron microscope or a semiconductor inspection apparatus such as CD SEM or DR SEM, an electron source having a covering layer of zirconium and oxygen formed on a tungsten single crystal needle 1 having <100> orientation, i.e. a ZrO/W Schottky electron source, is widely used, since it has a high brightness and a long life. Further, in such a device, it is common to employ a low acceleration electron beam of at most 1 kV for the observation and measurement of a test sample as it is.
In a case where a low acceleration electron beam is used, the diameter of the electron beam demagnified by a lens, is governed by a chromatic aberration (e.g. J. Pawley, Journal of Microscopy, 136, Pt1, 45 (1984)). In order to reduce such a chromatic aberration, it is necessary to reduce the energy spread of electrons emitted from the electron source. The energy spread of the Schottky electron emission source will not be lower than 2.45 KBT even at the minimum. Here, KB is a Boltzmann constant, and T is the absolute temperature at the electron emission area “R. D. Young, Phys. Rev. 113 (1959) p 110”. Accordingly, it is effective to lower the operation temperature of the electron source in order to reduce the chromatic aberration, but on the other hand, in the Schottky electron emission or thermionic emission, when the operation temperature is lowered, the emission current dramatically decreases. Therefore, in order to lower the operation temperature of the electron source, an electron source having a low work function must be used. From the foregoing viewpoint, a research for an adsorption species on a tungsten single crystal having a low work function relating to a ZrO adsorption layer, and on its supply source has been actively conducted in recent years (e.g. H. Nishiyama, T. Ohshima, H. Shinada, Applied Physics, Vol. 71, No. 4 (2002) p 438, or H. Nishiyama, T. Ohshima, H. Shinada, Applied Surface Science 146 (1999), p 382, or Y. Saito, K. Yada, H. Ando, Shingakugihou ED2001-175 (2001-12) p 15).
On the other hand, a disperser cathode, a L-type cathode or an impregnated cathode has been known since long ago, as an electron source for a cathode ray tube, wherein barium oxide, barium carbonate, or barium oxide and calcium oxide, aluminum oxide or the like, are added to tungsten, followed by sintering, to form an adsorption layer of Ba or BaO on the surface of a tungsten sintered body thereby to lower the work function (e.g. A. H. W. Beck, The Institution of Electrical Engineers, Paper No. 2750R, November 1958, p 372, as summarized on p 378-381).
An impregnated cathode is operated at a temperature of from about 1,000 to 1,300 K. Accordingly, by forming a BaO adsorption layer on a tungsten single crystal by a similar method, the reduction of the work function will proceed. Thus, it is easily expected that electron emission with a low energy spread can be carried out by the operation at a temperature of from about 1,000 to 1,300 K.
Apart from the energy spread in the operation at a low acceleration voltage, attention is drawn to an operating angular intensity in an application wherein the throughput is of importance. A high throughput is required in an application to an electron beam lithography apparatus or DR SEM. In such an application, a ZrO/W Schottky electron source is operated at a high angular current density of about 0.4 mA/sr, and a HfO/W Schottky electron source has been proposed to meet the requirement for a higher angular current density operation (JP-A-2001-319559).
An electron source is known wherein a barium-supplying source made of barium oxide or (Ba, Sr, Ca) oxide, is formed at a portion of tungsten single crystal to let barium diffuse on the surface of the tungsten single crystal thereby to lower the work function to a level of 1.2 eV, so that the electron source is operated at a low temperature of about 1,000 K. Although not directly verified, this electron source is considered to have a BaO adsorption layer on a tungsten single crystal (H. Nishiyama, T. Ohshima, H. Shinada, Applied Physics, Vol. 71, No. 4 (2002) p 438, or JP-A-10-154477). In this study, it is reported that in a case where a BaO adsorption layer is formed on a needle of tungsten single crystal having <100> orientation by means of a (Ba, Sr, Ca) oxide, in its operation at 1,000 K after heat treatment at a temperature of at least 1,500 K, the electron emission is confined at a narrow angle along the emission axis, thus showing preferred emission characteristics as an electron source. However, on the other hand, it is stated that the time for stable operation is very short at a level of a few hours, and it will be required to carry out thermal treatment at a temperature of at least 1,500 K repeatedly, which is considered to be not practically useful for industrial application. Further, a case wherein barium oxide is used as a supply source, has also been reported, but in such a case, it is pointed out that the electron emission will be of four fold symmetrical and can not uniformly be confined along the emission axis, and the reproducibility is poor.
JP-A-2001-319559 discloses a HfO/W Schottky electron source whereby at an operation angular intensity of 1.0 mA/sr, the total emission current is at most 350 μA, but in recent years, a still higher angular intensity operation (from 3 to 5 mA/sr) has been required. In such an extremely high angular intensity operation, the total emission current will also be high. Accordingly, electrons will be impinged on an extractor electrode or a metal aperture on the electron beam emission axis, whereby it frequently occurs that the outgasing will be remarkable, and ions formed by a collision of electron rays with the gas, will bombard the electron source to give a damage thereto, or will cause an arc discharge thereby to destroy the electron source.